Dhuum's Release is Inevitable

[draxynnic]

Actually, it is outright stated, in the Terrorweb Queen quest:

Originally Posted by Reaper of the Spawning Pools

The source of this trouble draws near. The more we turn the tide of battle against the demons, the more desperate they become. It will not be long before a Terrorweb Queen arrives, anxious to strike down that which has destroyed so much of her brood. When she arrives, you must destroy her.

(if asked about the quest): If you can destroy the Terrorweb Queen, it should stall the demon's attack for a time. Charge forth into the heart of the demon spawning pools and see if you can draw out the Queen.